This invention relates to a wireless communication device and a method of controlling a searcher in a CDMA (Code Division Multiple Access) system.
A recent attention tends to be focused on a CDMA system as a wireless or radio communication system which carries out communication between a base station and a plurality of mobile terminals by a wireless or radio signal within a service area or cell. This is because communication between the base station and a plurality of mobile terminals can be carried out by the same frequency within the same cell and, as a result, a frequency resource can be effectively used in the CDMA system.
As such a CDMA system, a direct sequence (DS)-CDMA system is known which directly modulates an information signal into a spread signal by a specific spread spectrum code peculiar to each mobile terminal. The DS-CDMA system makes it possible to spread the information signal into a very wide band. More specifically, communication from the base station to each mobile terminal is carried out through a forward link while communication from each mobile terminal to the base station is carried out through a backward or reverse link.
In general, the forward link includes a pilot channel and a traffic channel while the backward link includes a traffic channel without any pilot channel. At any rate, a receiver section of the base station and the mobile terminal must identify each channel from one another to detect a counterpart side on the communication.
In addition, it is to be noted in the CDMA system that the receiver section of each of the base station and the mobile terminal is given reception signals through different propagation paths which may be collectively called a multi-path. Therefore, the reception signals are inevitably subjected to interference on passing through the different propagation paths. Under the circumstances, the receiver section should reliably detect its own channel from the reception signals received through the multi-path. Taking this into consideration, the receiver section in the CDMA system is equipped with a searcher or searchers for searching its own channel which is arranged within a time slot predetermined for the receiver section.
In the interim, the above-mentioned DS-CDMA system includes a mobile communication system of DS-CDMA which is implemented in compliance with the IS 95 standard prescribed by ANSI (American National Standards Institute). In the mobile communication system, communication from each mobile terminal to the base station is carried out through the reverse or upward link by using a sequence of burst like frames from each mobile terminal. This mobile communication system is helpful to save power consumption in each mobile terminal.
Specifically, each of the frames is divided into sixteen slots for arranging an audio data signal or any other data signal given in the form of a digital signal. In this event, the digital signal is usually obtained by encoding a sound signal into digital samples by a codec and by carrying out predetermined code conversion about the digital samples by a vocoder included in the mobile terminal. Herein, it is to be noted that the digital samples are converted by the vocoder into different rates of the digital signal, as an amount of the digital samples is varied in each frame.
The digital signal of different rates is arranged in each frame at different frame rates determined by the number of data transmission slots in each frame. As a rule, the frame rates are classified into four rates, namely, a full rate, a half rate, a quarter rate, and an eighth rate. The digital signals are arranged in all of the slots in each frame at the full rate while the digital signals are arranged in half slots of sixteen slots at the half rate. Likewise, the quarter and the eighth rates are for arranging the digital signals in four and two slots included in each frame, respectively.
In U.S. Pat. No. 5,535,239 issued to Padovani et al, disclosure is made about a data burst randomizer which is used in a transmission system and which serves to determine power control groups in accordance with a predetermined algorithm by monitoring predefined bits in a previous frame. However, Padovani et al never point out any problem which might occur on reception of the digital signals of variable frame rates.
Practically, when such digital signals of variable frame rates are received by a conventional receiver section, a searcher in the conventional receiver section is put into an active state, regardless of the frame rates of a reception signal. In other words, the searcher is always operated in tune with the full rate. Accordingly, when transmission is made at any other rates than the full rate, the searcher in the receiver section is uselessly operated and gives rise to a useless operation of any other peripheral circuits related to the searcher for a quiescent period namely, an unvoiced period. This shows that probability of searching an unvoiced slot in the conventional searcher is determined by a ratio of the voiced period to a whole time.
Moreover, such a useless searching operation in the searcher brings about deteriorating a quality of communication due to interference and noise caused to occur in the unvoiced period or slots. To the contrary, it fears that a serious loss of data takes place owing to misdetection of a desired slot.
Herein, consideration will be made about operation which is carried out in a mobile terminal during soft handoff procedure from a current base station to a new base station. In this event, the new base station can not detect a location of the mobile terminal before demodulation of a data signal arranged in the traffic channel, because of absence of any access channel in the reverse link. During the handoff procedure or operation, it is very important to accurately acquire a desired slot and to detect a data signal arranged in the traffic channel of the desired slot, so as to maintain the quality of communication and to avoid a call drop.
As mentioned above, when a searching operation of the searcher is executed in connection with slots which include no data, superfluous interference takes place not only during the soft handoff procedure but also during a normal communication, which results in degradation of the quality of communication.
In order to solve the above-mentioned problem, the instant inventor has already proposed a searcher control method and device in Japanese Unexamined Patent Publication No. 2000-165351 (the corresponding U.S. patent application Ser. No. 09/449,548). In the proposed method and device, burst candidate slot information is calculated from a current transmission data signal and is used to control a searcher or searchers. In this event, the burst candidate slot information is determined by a data burst randomizer (DBR2).
However, the method and device are disadvantageous in that slots with data bursts might not be searched and a long time is often required to acquire paths by the searcher or searchers when strong fading or the like take place in a communication environment.
At any rate, no consideration is made about an influence of a delay time introduced before the searcher or searchers. Specifically, a frame rate signal can be detected by a receiver of the device to specify each frame rate and is sent to the DBR to control the searcher or searchers. Since several frames are required to detect the frame rate signal, searching operation of the searcher or searchers is inevitably delayed from a latest received frame. As a result, the method and device described in the above-mentioned application can realize neither fast acquisition of the paths nor accurate prediction of the frame rate.